Up to now, some image reading apparatuses provided in digital copying machines, printers, facsimile apparatuses and the like are equipped with an openable and closable original treatment apparatus which is an example of a sheet transport apparatus for automatically feeding an original in the form of a sheet or the like to the image reading apparatus. The image reading apparatus performs image reading with an exposure means on the original automatically fed by the original treatment apparatus.
As a conventional example of the exposure means, an image reading method (hereinafter referred to as flow reading) is known according to which an exposing device is fixed at a predetermined reading position on a platen glass plate provided in an image reading apparatus, the image being read by moving the original past the reading position at a predetermined speed. This flow reading is advantageous in that one original can be quickly replaced by another, and is applied to a variety of products.
However, in an original treatment apparatus utilizing the flow reading technique, in which the original is read while being transported, the original may be raised or flutter vertically during reading under the influence of the guide load or guide curvature. Thus, the original may be rather unstable.
As an example of the means for coping with this, a construction is available in which a platen roller is provided opposite to the platen glass plate in order to press the original against the platen glass plate.
FIG. 15 shows an example of a conventional image reading apparatus. The image reading apparatus shown in FIG. 15 is equipped with an original glass stand 300 for a reading system (hereinafter referred to as the book reading system) in which a stationary original placed thereon is read, a flow reading glass 301 for the flow reading system, and a reading sensor 302 consisting of a contact image sensor or the like which is an image reading means used for both reading systems.
In the flow reading system, a platen roller 304 arranged above the flow reading glass 301 at a predetermined distance therefrom is rotated at a predetermined speed, whereby at the time of flow reading, the original is transported in a stable manner while restricting the amount by which the original is raised from the upper surface of the flow reading glass 301.
The flow reading system using the platen roller 304 has a problem in that when there is a variation in the reading position, i.e., the stop position of the reading sensor 302, with respect to the platen roller, a variation in the brightness level of the read image is involved. FIG. 16 is a schematic view of a conventional image reading apparatus for explaining the cause of this phenomenon.
In FIG. 16, the route of light from an illuminating means 305 in the case of the book reading system is designated by the reference sign 16A, and the route of light from the illuminating means 305 in the case of the flow reading system is designated by the reference sign 16B. In FIG. 16, the route of light applied from the illuminating means on one side and transmitted through the original is schematically indicated by double-dotted lines. In the example shown, the reading position for the flow reading system is deviated from the position directly below the center of the platen roller 304 to the upstream side with respect to the original transporting direction.
In the case of the book reading system designated by the reference sign 16A in FIG. 16, the light transmitted through the original 306 is reflected in the direction of the reading sensor 302 by a white pressure plate 307 pressing the original 306 against the original glass stand 300, and illuminates the back surface of the original. In contrast, in the case of the flow reading system designated by the reference sign 16B in FIG. 16, a part of the light transmitted through the original 306 and reflected by the white platen roller 304 is reflected away from the reading sensor 302 due to the curvature of the platen roller 304.
As a result, the quantity of light illuminating the back surface of the original 306 is reduced, resulting in a reduction in brightness when the reflected light from the original is read by an image sensor 309 through a lens 308. As the reading position is shifted away from the position directly below the center of the platen roller 304, the quantity of light reflected in the direction of the reading sensor 302 decreases, resulting in a further reduction in the brightness of the image even when the same original is read. The higher the light transmittance of the original 306, the greater the influence of this phenomenon. Thus, the thinner the original 306, the more conspicuous is the reduction in brightness.
To perform image reading without being influenced by this reduction in brightness, it is necessary to accurately place the reading sensor 302 at the position directly below the rotation center of the platen roller 304, in which it is possible to obtain substantially the same brightness as in the case of the book reading system.
However, due to the accuracy in the stop position of the reading sensor 302, play or slop, etc. in the hinge portion of the ADF 303 mounted so as to allow opening and closing, it is difficult to perform accurate positioning on the reading sensor 302 with respect to the platen roller 304.
Further, the portion of the surface of the flow reading glass 301 directly below the center of the platen roller 304 and the portion thereof on the downstream side in the original transporting direction are subject to adhesion of dirt. When dirt exists at the reading position, the image will be streaked. In view of this, it is desirable for the reading position to be set somewhat upstream with respect to the position directly below the center of the platen roller 304.
To prevent generation of a streaked image due to dirt on the flow reading glass 301, there has been proposed a method according to which image reading is performed at the reading position for flow reading while rotating the platen roller 304, with the original not being transported, to thereby distinguish between dirt on the flow reading glass 301 and dirt on the surface of the platen roller 304. When dirt is detected on the flow reading glass 301, the reading position is shifted. In the case in which this operation for preventing generation of a streaked image is performed, the reading position for flow reading is inevitably moved.
As described above, due to the influence of the component accuracy and the operation for preventing generation of a streaked image, it is difficult for the reading position for flow reading to remain the same always with respect to the platen roller 304. Thus, a variation in brightness as mentioned above is involved.
FIG. 17 shows another example of the construction of the portion of an original treatment apparatus using a platen roller which is in the vicinity of the reading position. In the vicinity of the reading position of this conventional apparatus using a platen roller, there are provided transport roller pairs 402 and 403 situated on the upstream and downstream sides of the reading position 401 and adapted to transport the original at a predetermined speed, and a platen roller 405 situated substantially above the reading position and rotatable by a driving means (not shown).
An exposure device 409 is provided below a platen glass plate 404 to perform image reading operation on an original transported at a uniform velocity between the platen roller 405 and the platen glass plate 404. To stabilize the quantity of light on the exposure device 409 during reading to thereby prevent show-through, and to realize contrast enhancement on the characters and background when reading an original likely to be see-through as in the case of a thin sheet or an intermediate paper, the color of the platen roller 405 is white. Thus, the range in which reading by the exposure device 409 is possible must be a range in which the whiteness of the platen roller 405 is suited for reading. In view of this, the exposure device is usually provided directly below the platen roller.
Further, the platen roller 405 is supported by an oscillation arm 406 so as to be capable of oscillating about a fulcrum shaft 407 (which, in this example, also serves as the shaft of one roller of the transport roller pair 402), and is pressurized downwards by an urging member 408. Due to this arrangement, when the original treatment apparatus is closed, the oscillation arm 406 abuts against the platen glass plate 404 and follows the reading position 401 through equalization.
If at this time the platen roller 405 were brought into contact with the platen glass plate 404, it would be constantly rubbed against the platen glass plate 404, thereby damaging the glass. In view of this, the outer diameter of the oscillation arm 406 is larger than the outer diameter of the platen roller 405, forming a gap X between the platen glass plate 404 and the platen roller 405.
By minimizing this gap X, rising of the original during transport is prevented so as to effect image reading at an appropriate level of brightness. At this time, the platen roller 405 is being driven, so that it is possible to transport the original without imparting any load thereto. In particular, in the case in which a contact image sensor is used for the exposure device 409, the reading depth is small, so that in many cases the platen roller 405 used is one capable of easily minimizing the gap X.
The conventional original treatment apparatus performing flow reading using a platen roller has the following problem: In order to stabilize the light quantity on the exposure device during reading to prevent show-through, and to effect contrast enhancement on the characters and background when exposure is effected for the original likely to be see-through as in the case of a thin sheet or an intermediate paper, a white member facing the back surface of the original is provided in the portion of the image reading apparatus on the side opposed to the reading position. In this regard, the original treatment apparatus using a platen roller is no exception, and is normally equipped with a white platen roller opposed to the reading position.
In the construction using a platen roller, due to the curvature of the platen roller, if the reading position is deviated from the position directly below the platen roller, the distance between it and the roller increases, resulting in an insufficient light quantity. This can also cause the shadow of the gap between the platen roller and a guide in the vicinity thereof to be reflected, with the result that the whiteness of the background deteriorates, with the background being read rather dark.
An attempt to perform reading in a place free from such problems would lead to a restriction in the range in which reading is possible in the case of an original treatment apparatus using a platen roller. Similarly, when a contact image sensor (CIS), whose reading depth is less than that of a CCD, is used, the influence of the curvature of the platen roller will be still greater, which means the range in which reading is possible is restricted to the portion in the vicinity of the position directly below the roller.
In view of this, the reading position has conventionally been adjusted to a position in which appropriate reading is possible through positional adjustment of the platen roller and the exposure device by using a jig, detection means, and the like.
It might be possible to widen the reading area by making the curvature gentler by making the diameter of the white roller as large as possible. However, given the component variation of the original treatment apparatus and of the exposure device, it would be necessary to use a white platen roller with a considerably large diameter, resulting in an increase in apparatus size and an increase in cost.